ESQL: Fix usage of already released null block in ValueSourceReaderOperator

docs/changelog/126411.yaml

@@ -0,0 +1,5 @@
+pr: 126411
+summary: Fix usage of already released null block in `ValueSourceReaderOperator`
+area: ES|QL
+type: bug
+issues: []

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/ValuesSourceReaderOperator.java

@@ -681,7 +681,12 @@ public BlockLoader.Builder nulls(int expectedCount) {
 
         @Override
         public Block constantNulls() {
-            if (nullBlock == null) {
+            // Avoid creating a new null block if we already have one.
+            // However, downstream operators take ownership of the null block we hand to them and may close it, forcing us to create a new
+            // one.
+            // This could be avoided altogether if this factory itself kept a reference to the null block, but we'd have to also ensure
+            // to release this block once the factory is not used anymore.
+            if (nullBlock == null || nullBlock.isReleased()) {
                 nullBlock = factory.newConstantNullBlock(pageSize);
             } else {
                 nullBlock.incRef();

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -947,7 +947,12 @@ public enum Cap {
         /**
          * Supercedes {@link Cap#MAKE_NUMBER_OF_CHANNELS_CONSISTENT_WITH_LAYOUT}.
          */
-        FIX_REPLACE_MISSING_FIELD_WITH_NULL_DUPLICATE_NAME_ID_IN_LAYOUT;
+        FIX_REPLACE_MISSING_FIELD_WITH_NULL_DUPLICATE_NAME_ID_IN_LAYOUT,
+
+        /**
+         * Fix https://github.com/elastic/elasticsearch/issues/125850
+         */
+        FIX_DOUBLY_RELEASED_NULL_BLOCKS_IN_VALUESOURCEREADER;
 
         private final boolean enabled;
 

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/esql/100_bug_fix.yml

@@ -335,3 +335,58 @@
       esql.query:
         body:
           query: 'FROM test_grok | KEEP name | WHERE last_name == "Facello" | EVAL name = concat("1 ", last_name) | GROK name "%{NUMBER:foo} %{WORD:foo}"'
+---
+"union types with null blocks from missing fields #125850":
+  - requires:
+      test_runner_features: [allowed_warnings_regex, capabilities]
+      capabilities:
+        - method: POST
+          path: /_query
+          parameters: []
+          capabilities: [fix_doubly_released_null_blocks_in_valuesourcereader]
+      reason: "fixed handing out already closed null block references in ValueSourceReader"
+  - do:
+      indices.create:
+        index: test1
+        body:
+          mappings:
+            properties:
+              truefalse1 :
+                type : boolean
+              truefalse2 :
+                type: boolean
+  - do:
+      indices.create:
+        index: test2
+        body:
+          mappings:
+            properties:
+              truefalse1 :
+                type : keyword
+              truefalse2 :
+                type: keyword
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - { "index": { "_index": "test1" } }
+          - { "truefalse1": null}
+          - { "index": { "_index": "test2" } }
+          - { "truefalse1": null }
+
+  - do:
+      allowed_warnings_regex:
+        - "No limit defined, adding default limit of \\[.*\\]"
+
+      esql.query:
+        body:
+          query: 'FROM test* | eval t1 = truefalse1::boolean, t2 = truefalse2::boolean | keep t1, t2'
+  - match: { columns.0.name: t1 }
+  - match: { columns.0.type: boolean }
+  - match: { columns.1.name: t2 }
+  - match: { columns.1.type: boolean }
+  - length: { values: 2 }
+  - match: { values.0.0: null }
+  - match: { values.0.1: null }
+  - match: { values.1.0: null }
+  - match: { values.1.1: null }